The present invention relates to an improved control system utilizing thyristors for controlling a D.C. electric motor.
Heretofore, static Ward-Leonard systems have been employed as rotation speed control systems for electric motors. One embodiment of such a rotation speed control system is shown in FIG. 1. The conventional rotation speed control system, as shown in FIG. 1, includes a three-phase transformer for supplying A.C. power for which the phases are designated by reference characters R, S and T, respectively; power supply-side reactances 2 each having a reactant value of L.sub.1 ; static Ward-Leonard side A.C. reactors L.sub.3 each having a reactance value of L.sub.2 ; a thyristor converter including thyristors 4a through 4f; a D.C. reactor 5; an armature 6 of a D.C. electric motor M; and a field winding 7. Generally, while a thyristor converter 4 including a pair of forward and reverse thyristor converters is customarily employed, in order to simplify the schematic diagram shown in FIG. 1, there is shown only one of the forward and reverse thyristor converters. This conventional rotation speed control system is well-known in the art and therefore a description of the operation thereof is omitted.
FIG. 2 is a schematic diagram showing a waveform of a power supply voltage appearing at a point A on an input side of the thyristor converter 4 shown in FIG. 1 with the thyristor firing angle being approximately 90.degree.. In FIG. 2, reference character T represents a period of time corresponding to a commutation overlapping angle. The depth of the voltage notch at the time of commutation is defined by the ratio of L.sub.1 to L.sub.2. Accordingly, the following expression can be obtained. EQU (E.sub.1 /E.sub.0)=(L.sub.2 /L.sub.1 =L.sub.2),
where E.sub.0 is the maximum voltage value and E.sub.1 the notch voltage value.
In the case where A.C. power is supplied by an emergency generator during an interruption of the normal power source, due to the fact that the power supply-side reactance value L.sub.1 of the emergency generator is relatively large, the notch voltage E.sub.1 becomes very small and consequently, the depth of the voltage notch becomes quite deep. In addition, assuming that a circuit element, and more particularly a circuit element including a uni-junction transistor sensitive to the voltage notch, is coupled to the point A, the voltage notch causes erroneous operation of the circuit element.
In order to make the voltage notch smaller, as is clear from the above-mentioned expression, the reactance value L.sub.2 is made larger. In this case, however, this brings about an accompanying disadvantage in that the size of the A.C. reactor 3 is increased resulting in an increase in the communication failure of the thyristors 4a through 4f and the like.
FIG. 3 is a schematic diagram showing a second conventional control system for a D.C. electric motor which was developed to eliminate the above-noted drawbacks. In FIG. 3, coupled to the input side of the thyristor converter 4 is a capacitor 8 provided for eliminating the above-described voltage notch. However, resonance may be caused by a combination of the reactances L.sub.1 and L.sub.2 and the electrostatic capacitance C. The resultant voltage at the input side of the thyristor converter 4 has a waveform as shown in FIG. 4. Accordingly, the circuit element coupled to the point A is subjected to a noise component included in the resultant voltage.